1. Field of the Invention
The invention is directed to an electrically operated drive-in tool having a ram for driving in a fastening element and supported in a guide for displacement therein, a drive unit for driving the ram and having at least one flywheel driven by an electric motor, and a return device for returning the ram in its initial position upon completion of the drive-in process. In drive-in tools of this type, fastening elements are driven into a substrate by a driving movement of the driving ram.
2. Description of the Prior Art
In the electrically operated drive-in tools, the driving ram is accelerated by at least one flywheel which can be driven by an electric motor. In drive-in tools which draw their energy from a battery, the driving energy is approximately 35 J to 40 J. According to the flywheel principle realized in the drive-in tools, the energy stored in the flywheel is transmitted to the driving ram by a clutch. This clutch must be switched very quickly and must transmit a very high energy within a very short time. Further, the clutch must be switched off again very quickly at the end of the drive-in process. The driving piston is moved back at the end of the drive-in process, e.g., by a spring element or a flexible belt.
U.S. Pat. No. 4,721,170 discloses an electric drive-in tool in which the driving ram is guided through between a flywheel, which is driven by an electric motor, and an idler wheel. A flexible belt which acts on the driving ram, on one hand, and is held at the housing, on the other hand is provided for returning the driving ram to its initial position after a drive-in process has been completed.
However, returning the driving ram with a flexible belt is only practical in drive-in tools with low setting power. In drive-in tools with higher driving energies for the driving ram greater than about 35 J, the life of the flexible belt is drastically reduced.
U.S. Pat. No. 4,129,240 discloses another electric drive-in tool in which a driving ram can be driven by a flywheel which is driven by an electric motor. A return device for the driving ram comprises a return wheel that is held at a supporting arm. This return wheel is switchable between an active position, in which it is driven by a shaft of the flywheel and engages the driving ram, and an inactive position in which it is lifted from the driving ram and is not driven by the shaft of the flywheel.
This is disadvantageous in that the mechanism for switching the return wheel is very complicated and slow.
Further, it is known from U.S. Pat. No. 4,129,240 to return the driving ram with a return spring which engages the housing, on one hand, and the driving ram, on the other hand. The drawback of the device of this U.S. Patent consists that the mechanical return spring does not have the required useful life in drive-in tools with a setting power greater than about 35 J. Further, the mass of the spring leads to drastic energy losses.